Macrophage activation syndrome (MAS) is a serious, potentially fatal complication of Systemic Juvenile Idiopathic Arthritis. In clinical practice, there is a strong need for reliable biomarkers that would identify patients at risk for this complication. MAS is caused by the exaggerated inflammatory response involving mainly two types of immune cells - macrophages and T lymphocytes. In clinically similar genetic conditions the development of the exaggerated immune response has been linked to defective function of cytolytic cells. These cells typically kill abnormal cells such as tumor cells or cells infected with viruses. There is some evidence that cytolytic cells may also be involved in the elimination of overly activated immune cells. Therefore, if they do not function properly, the immune response may not be terminated in a timely manner. This would lead to uncontrolled inflammation seen in MAS. Previously, we have identified several genetic markers within the MUNC13-4 gene inherited as a single haplotype. Since MUNC13-4 protein is involved with the cytolytic function this observations is important. An important unanswered question is whether the presence of the haplotype in the MUNC 13-4 is associated with abnormal function of the MUNC13-4 protein and, thus, directly contributes to the development of cytolytic dysfunction seen in patients MAS. Another possibility is that the described haplotype may extend either upstream or downstream of the MUNC13-4 gene and involve additional polymorphisms in the neighboring immunologically relevant genes. These two possibilities will be explored in the proposed study. PUBLIC HEALTH RELEVANCE: Macrophage activation syndrome (MAS) is a serious, potentially fatal complication of Systemic Juvenile Idiopathic Arthritis (sJIA) characterized by an overwhelming inflammatory reaction driven by excessive activation and expansion of T cells and hemophagocytic macrophages. Clinically, MAS is similar to the autosomal recessive disorder Familial Hemophagocytic Lymphohistiocytosis (FHLH). In FHLH, the uncontrolled proliferation of T cells and macrophages has been associated with decreased NK cell and cytotoxic cell functions secondary to mutations either in the gene encoding perforin or in the MUNC13-4 gene whose product is involved in the intracellular transport of perforin. Our observations suggest that as in FHLH, sJIA patients who develop MAS, have profoundly depressed NK function. We have recently reported an association with single-nucleotide polymorphisms (SNPs) in the MUNC13-4 gene inherited as an extended haplotype in the majority of MAS patients. This haplotype is rare in healthy controls or in patients with sJIA without MAS. Moreover, a statistically significant increase in the frequency of the haplotype was also noted in patients with Reactive HLH. An important unanswered question is whether the observed polymorphism in the MUNC 13-4 is associated with abnormal function of the MUNC13-4 protein and, thus, directly contributes to the development of cytolytic dysfunction seen in patients MAS. Another possibility is that the described haplotype may extend either upstream or downstream of the MUNC13-4 gene and involve additional polymorphisms in the neighboring immunologically relevant genes. To address these questions we propose to genotype sJIA/MAS patients and controls for additional 64 SNPs located upstream and downstream of the MUNC13-4 gene to determine whether the haplotype of interest extends up- or downstream of the gene and includes additional SNPs either in the promoter region of the MUNC13-4 gene or in the neighboring genes. If this is the case, additional sequencing may be necessary to identify additional functionally significant SNPs. In parallel, we will determine whether the presence of the haplotype is associated with either abnormal function of the MUNC13-4 protein or changes in the levels of expression of the genes located upstream or downstream of the MUNC13-4 gene. The long-term goal of this avenue of research is to define genetic defects responsible for the development of MAS in sJIA .